In a marine jet propulsion unit, water is taken in through an inlet in the bottom of the hull and is accelerated rearwardly in the unit for discharge at the stern. The craft is propelled forward by reaction to the rearward acceleration of the mass of pumped water passing through the unit, and therefore the propulsion force depends upon the rate of flow of pumped water per unit time and the rate of change of momentum imparted to the pumped water in the course of its flow from the intake to the discharging outlet. It will be apparent that if the water, in passing through the unit, acquires a certain amount of kinetic energy from the impeller, and then gives up a part of that kinetic energy during subsequent passage through straightening vanes or other flow redirecting means, the reaction forces imposed upon the flow redirecting means may oppose the propulsion reaction force and correspondingly reduce the propulsion efficiency of the unit. In like manner the water entering the inlet may have a substantial ram velocity due to the forward speed of the craft, and if it gives up a part of that velocity before encountering the impeller, as by flow disturbance due to the impeller shaft or improper flow direction with respect to the first impeller blading, much of this ram benefit is lost. Ideally, therefore, the water passing through a marine jet propulsion unit should flow from intake to outlet in a straight, substantially nonturbulent and unswirling but steadily accelerated stream.
Water entering the inlet of a hydraulic jet propulsion unit is at atmospheric pressure, and it is discharged to atmospheric pressure at the outlet. Because of the limited atmospheric "push" at the intake, the rise in pressure along the flow path of the pumped water must be gradual and at a relatively small rate to avoid cavitation with its attendant high internal losses, low efficiency and reduction of component life. The art has understood for some years that if energy could be added to the pumped water in the form desired for propulsion--i.e., kinetic energy--with only a slight increase in pressure of the water between the intake and the outlet, the unit could operate at high speed without danger of cavitation and its degenerative effects.
Prior hydraulic jet propulsion units have almost invariably comprised propeller-like or turbine-like impellers which generated pressure largely as a function of their rotational speed and outside diameter. To avoid abrupt changes in pressure along the flow path of the pumped water these were often arranged to provide multiple stages, as disclosed for example in the applicant's prior U.S. Pat. No. 3,328,961 and No. 3,405,526. Such an impeller induced a swirling flow that had to be straightened by fixed vanes downstream from each impeller stage, and it was obvious that a certain amount of energy was wasted in inducing the swirl and again in straightening the flow.
A proposed impeller intended to avoid these deficiencies, disclosed in the applicant's U.S. Pat. No. 3,183,878, comprised a generally conventional sliding vane pump that had its axis horizontal. Pumped water flowed upwardly and rearwardly from the intake to the pump housing along a gently curving path, and then flowed substantially straight rearwardly from the pump housing to the discharging outlet. In theory the unit disclosed in that patent provided for a nonswirling flow of pumped water. However, the centrifugally responsive radially sliding vanes of the pump rotor had to be confined within a cylindrical pump housing wall that was circumferentially continuous, and therefore the pumped water had to enter and leave the pump housing through ports in its cylindrical wall. If these ports were of substantial width, as measured parallel to the pump axis, they left little wall area to support the vanes as they passed across the ports and thus subjected local portions of the vanes to frictional wear that could result in destruction of the pump. Pump life could be prolonged by reducing the area of the ports, but at the cost of constricting the flow of water through each port, thus entailing an inevitably unsatisfactory compromise between operating efficiency and pump life. Another important disadvantage of a sliding vane impeller is that it is likely to be destroyed by particles of foreign matter in the fluid passed through it, whereas particulate material of one kind or another will inevitably be drawn into a marine jet propulsion unit from time to time unless it has a filter at its intake that sacrifices propulsion efficiency in favor of effective filtration.
As prior art that might be considered to have some pertinence to the present invention, reference can be made to U.S. Pat. No. 3,965,846 to Mihara, No. 745,732 to Little, and No. 3,292,899 to Egli.
Mihara and Little disclose impeller units comprising endless belts that move in laterally adjacent horizontal orbits and have laterally projecting blades. In both cases the impeller units generate propulsive force by their action on unconfined water outside the hull of the craft, rather than by acting upon a confined stream of water; hence these are not jet propulsion units but are more nearly related to paddle wheel propellers. Unconfined water acted upon by a paddle wheel tends to have substantial components of lateral flow which do not give rise to forward reaction forces that are useful for propulsion.
Egli discloses an energy transfer machine wherein an endless belt having laterally extending vanes has one straight stretch which passes through a housing, entering and leaving the housing through ingress and egress passages that are substantially sealed by the belts and vanes. Fluid moves through the housing from an inlet to an outlet that are respectively separate from the ingress and egress passages, and within the housing it zigzags across the path of the vanes, which are cambered to promote such transverse flow. Because the fluid flow path through the housing is a sinuous one, it is apparent that much of the energy transferred from the endless belt to the fluid is manifested as static pressure rather than as kinetic energy. Furthermore, the arrangement poses obvious problems with respect to the possibility of undesired issuance of fluid from the egress passage.